This diagram shows the ingredients that make up the universe. Astronomers now realize that the universe's main ingredient is "dark energy," a mysterious form of energy that exists between galaxies. The next largest constituent is dark matter, which is an unknown form of matter. The rest of the universe consists of ordinary matter. Most of it is locked up in stars and coluds of gas. A tiny fraction of this matter is composed of heavier elements, the stuff of which humans and planets are made.